Message-ID: <a1a11072-908b-352e-a49f-49240d430df9@broadcom.com>
Date:   Mon, 6 Feb 2017 15:16:32 -0800
From:   Florian Fainelli <florian.fainelli@...adcom.com>
To:     Stephen Boyd <sboyd@...eaurora.org>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     Markus Mayer <markus.mayer@...adcom.com>,
        Markus Mayer <code@...yer.net>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Arnd Bergmann <arnd@...db.de>,
        Broadcom Kernel List <bcm-kernel-feedback-list@...adcom.com>,
        Linux Clock List <linux-clk@...r.kernel.org>,
        Power Management List <linux-pm@...r.kernel.org>,
        Device Tree List <devicetree@...r.kernel.org>,
        ARM Kernel List <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/2] dt-bindings: brcm: clocks: add binding for
 brcmstb-cpu-clk-div

On 02/06/2017 02:59 PM, Stephen Boyd wrote:
> On 02/03, Florian Fainelli wrote:
>> On 02/03/2017 12:06 PM, Stephen Boyd wrote:
>>> On 02/01, Markus Mayer wrote:
>>>> On 20 January 2017 at 16:52, Stephen Boyd <sboyd@...eaurora.org> wrote:
>>>>>
>>>>> Are these properties used? Please don't put these types of
>>>>> details in DT.
>>>>
>>>> Yeah, unfortunately they are. Luckily, I think the issue can be
>>>> resolved quite easily, because the user of those properties isn't
>>>> involved in this series.
>>>>
>>>> They are currently being used by a clock driver
>>>> ("drivers/clk/clk-brcmstb.c") that hasn't been upstreamed yet. I
>>>> performed some code archeology. While I wasn't 100% successful in
>>>> tracking down the origins of this interface, it looks like it was
>>>> designed this way a while back (4+ years or so), probably before
>>>> device tree best practices were developed or, at least, before they
>>>> were widely known.
>>>>
>>>> So, what I can do is to remove the properties from the official
>>>> binding. (I'll send an update to that effect shortly.) Once the
>>>> binding is accepted upstream, we can work on fixing up the design of
>>>> clk-brcmstb.c, so it doesn't rely on these properties anymore (and
>>>> derives them from the compatible string instead), and then proceed to
>>>> upstream that, as well.
>>>
>>> Ok. Sounds like some cleanup needs to be done on the way
>>> upstream.
>>>
>>>>> This register really looks like some offset in something larger.
>>>>> Is there some clock controller? What's the hw block at
>>>>> 0xf03e2000? Maybe I already asked this.
>>>>
>>>> It looks this way, but in this case, looks are deceiving. The address
>>>> and the length are really correct the way they are.
>>>>
>>>> This memory area holds a range of only loosely related configuration
>>>> registers. It's called the Bus Interface Unit Register Set and deals
>>>> with configuring the CPU in general. At address 0xf03e257c, there
>>>> happens to be the clock divider register we need, and it's really just
>>>> one register, i.e. 4 bytes.
>>>
>>> We've seen this style of hardware design before. I'd prefer we
>>> make the "Bus Interface Unit Register Set" into one device node
>>> and have a driver probe for it that registers this clock. If
>>> other things need to be controlled in there then the driver will
>>> do more than just register one clock, possibly hooking into
>>> multiple frameworks. The compatible string can indicate which SoC
>>> it is if the divider register offset changes or if the register
>>> layout is a total free for all.
>>
>> We already have another piece of drive code that manipulates registers
>> in the Bus Interface Unit located in drivers/soc/bcm/brcmstb/biuctrl.c
>> and which has little to nothing to do with the CPU's clock ratio. And
>> actually another one being submitted that deals with the CPU's
>> read-ahead cache. I would very much prefer we keep all of them separate
>> and dealing with just the register offset they need to do, but that does
>> not mean the Device Tree binding has to look that way though.
>>
>> The binding for the BIUCTRL register made it here:
>>
>> Documentation/devicetree/bindings/arm/bcm/brcm,brcmstb.txt
>>
>> so we should re-use that, and have a small piece of clock provided that
>> just uses the relevant register range within that larger register space
>> and provide the CLOCK_RATIO. Does that work?
>>
> 
> Ok. That's fine. The existing binding will be updated to include
> this new subnode then for the clock component?

Humm, I suppose we could do that yes, my original thought was to just
have this CPU clock provider remap the entire BIUCTRL register range
(of_iomap() etc.) and manipulate just the relevant register range of
interest (CPU_CLOCK_CONFIG_REG), but if you want a sub-node to appear,
we could probably do that as well.
-- 
Florian

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